1. Field of the Invention
The present invention relates to an optical information recording medium from which information is played back by the use of a light beam.
2. Description of Related Art
A compact disk (CD) is a typical example of an optical disk in which minute recesses called "pits" and corresponding to recorded information are formed in one surface of a transparent substrate. The recorded information is played back in accordance with changes in the quantity of reflected light produced by focusing a laser beam on the pits through the transparent substrate.
More specifically, the CD is standardized so that trains of pits having the shortest pit length of about 0.9 (.mu.m) are formed at a track pitch of 1.6 (.mu.m) on the transparent substrate which has a diameter of 120 (mm) and a thickness of 1.2 (mm). The CD is played back at a constant linear velocity of about 1.3 (m/s), and it has a recording capacity of about 650 (MB). Digital data comprising music, a picture or the like are stored in the CD. In the ensuing explanation herein, the phrase a "format conforming to the CD standard shall signify the above described specifications, and the term "CD" shall cover a CD-ROM (Compact Disk-Read Only Memory), or a Video-CD (Video-Compact Disk), etc.
On the other hand, research and development has been done on a digital video disk (DVD) which has a recording density which is about 6-8 times greater than a CD. At present, two design standards have been proposed as the DVD. One of the design standards is called an "SD" (Super-density Disk), and the other an "HDMCD" (High Density Multimedia Compact Disk). There is no interchangeability between both the design standards.
FIG. 3 is a model diagram showing the vertical sectional structure of the SD, while FIG. 4 is a model diagram showing the vertical sectional structure of the HDMCD.
As shown in FIG. 3, the SD is constructed so that a first transparent substrate 12, being 120 (mm) in diameter and 0.6 (mm) in thickness and formed with a first information surface 13, and a second transparent substrate 14, being 120 (mm) in diameter and 0.6 (mm) in thickness and formed with a second information surface 15 are adhered together by the use of an ultraviolet-curing resin 16. The information surfaces 13 and 15 of the respective substrates 12 and 14 are located inside the first and second transparent substrates. The front or outer sides of the first information surface 13 and the second information surface 15 are respectively covered with reflective films 17 made of a metal, such as aluminum, which reflects most of a laser beam. Recorded information items are played back from both the sides of the SD through the first transparent substrate 12 and the second transparent substrate 14.
An alternative SD construction has been proposed so that only the front or outer side of the first information surface 13 is covered with a semitransparent film made of a dielectric or the like instead of the reflective film 17. This alternative material is adapted to transmit part of the laser beam and to reflect the remainder thereof. As a consequence, both the information items recorded in the first information surface 13 and the second information surface 15 can be played back through the first transparent substrate 12.
The specifications of the SD presently proposed are a track pitch of 0.725 (.mu.m), the shortest pit length of 0.4 (.mu.m), a constant linear velocity of about 4 (m/s) in the playback mode, a storage capacity of about 10 (GB) using both information surfaces (13, 15), and a playback laser beam wavelength of either of 650 (nm) or 635 (nm). In the ensuing explanation herein, the phrase a "format conforming to the SD standard" shall signify the above described specifications.
On the other hand, as shown in FIG. 4, the HDMCD is constructed so that a transparent substrate 18 being 120 (mm) in diameter and 1.2 (mm) in thickness has two layers of information surfaces. The first information surface 19 is formed by injection molding, and the front or outer side thereof is covered with a semitransparent film 20 made of, e. g., a dielectric which transmits part of a laser beam and reflects the remainder. The second information surface 21 is formed so that an ultraviolet-curing resin 24, applied to a thickness of about 40 (.mu.m) on the semitransparent film 20, is hardened by irradiation with ultraviolet light while being pressed down by a stamper. The front or outer side of the second information surface 21 is covered with a reflective film 22 made of a metal, such as aluminum, which reflects most of the laser beam. Further, the front or outer side of the reflective film 22 is covered with a protective film 23 which is made of an ultraviolet-curing resin or the like.
The specifications of the HDMCD presently proposed are a track pitch of 0.84 (.mu.m), the shortest pit length of 0.451 (.mu.m), a constant linear velocity of about 4 (m/s) in the playback mode of this HDMCD, a storage capacity of about 7.4 (GB), and a playback laser beam wavelength of 635 (nm). In the ensuing explanation herein, the phrase a "format conforming to the HDMCD standard shall signify the above specifications.
As stated above, the optical disks SD, HDMCD and CD are based on different record formats. Moreover, the thickness of each transparent substrate is 0.6 (mm) in the SD, whereas the thickness of the transparent substrate is 1.2 (mm) in the HDMCD and CD. When the disks having the transparent substrates of the different thicknesses are to be played back using an identical objective lens, aberrations appear on the information surface, and the playback laser beam spot cannot be narrowed down to the diffraction limit that the pit information can be read.
Accordingly, the optical information recording medium in which the information has been recorded in the format conforming to the SD standard cannot be played back by a playback system adapted to the HDMCD or the CD standards. Likewise, the optical information recording medium in which the information has been recorded in the format conforming to the HDMCD or CD standard cannot be played back by a playback system adapted to the SD standard.